Cognitive Psychology: A Narrow Beam and a Rebuilding Crew

You are sure you remember where you parked, who was at the party, which of two cars cut you off. You can see it. Then a photo, a receipt or another person disagrees, and the picture you were so sure of turns out to have been assembled, a little at a time, after the fact.

Or: you are at a busy table, talking, and across the room someone says your name. You never heard the rest of that conversation, yet your own name pulled you out of this one. Or you try to keep a phone number in your head while a friend reads you another, and one of them simply vanishes.

Cognitive psychology is the study of those ordinary moments: how a mind takes in a flood of input, keeps a little of it, stores a fraction of that, and rebuilds the rest on demand. A graduate stops trusting the feeling of "I clearly remember" and starts asking how the memory was built. This matters in classrooms, in cockpits, and, as we will see, in courtrooms.

Attention is a narrow beam, and most of the world falls outside it

You are bombarded with far more than you can process, so the mind selects. In 1953 Colin Cherry played people two voices, one in each ear, and asked them to repeat one. They could, and could say almost nothing about the other voice, except that it was a voice, and that their own name, if it appeared there, often got through. Broadbent's 1958 filter model was the first answer: a bottleneck lets one channel through. Later models made the filter leakier, and the debate over how early or late the filter acts is still alive.

The vivid modern version is inattentional blindness. In the famous 1999 "gorilla" experiment by Daniel Simons and Christopher Chabris, viewers counting basketball passes often failed to notice a person in a gorilla suit walking through the scene; in most groups about half missed it. Nothing is wrong with their eyes. What you see is what you attend to, and you cannot tell what you are missing, because the missing is not marked.

The cleanest way to feel the limit is the Stroop effect (J. Ridley Stroop, 1935). Reading a word is so automatic that it intrudes when you try to name the colour of its ink. The conflict costs time you can measure.

Most people are slower on the conflicting trials, often by a tenth of a second or more, and make more errors. Nothing "loaded" more information; two automatic processes simply fought over one response. A graduate carries the lesson everywhere: attention is a single narrow beam that can be split only at a cost, which is why a phone call hurts driving even when the phone is hands-free.

Working memory holds about four chunks, not seven digits

Hold a phone number in your head and you are using working memory: the scratchpad where you keep things while you use them. George Miller's 1956 paper "The Magical Number Seven, Plus or Minus Two" made 7 famous. Later work, above all Nelson Cowan's 2001 review, argued the true limit is nearer four chunks once you stop people from rehearsing and regrouping. The word that matters is chunk: a unit that is one thing to you. The letters F-B-I-C-I-A-N-A-S-A are ten items, or three chunks if you know the acronyms.

Baddeley and Hitch (1974) described this scratchpad as a small set of parts, a loop for sounds, a sketchpad for pictures and a controller that directs attention. A graduate remembers the practical points. The store is small. It is easily overwritten. And the way out is not more effort but better chunking, which is why experts seem to have enormous memory in their own field, and ordinary memory outside it.

Most readers manage around six or seven plain digits and do noticeably better on longer strings when they are grouped, because three groups of three are three chunks. That is not a trick. It is the whole mechanism of memory improvement: the number stays small; the chunk gets bigger.

Encoding and retrieval: what goes in decides what comes out

Remembering has two halves. Encoding is how the information is processed on the way in; retrieval is how you get it back. In 1972 Fergus Craik and Robert Lockhart argued that deeper processing (what does this mean to me?) beats shallow processing (what does it look like?), and the result has held up well. A second idea, encoding specificity, says a cue works to the extent it matches how the memory was laid down. In a well-known 1975 study, Godden and Baddeley had divers learn word lists on land or underwater; words were recalled better in the place they were learned. The context effect is real but modest and not always found, so treat "study where you'll be tested" as a small bonus, not a rule.

And then there is time. Hermann Ebbinghaus (1885) measured his own forgetting of nonsense syllables and found that loss is fastest at first and then flattens. A 2015 replication of his work (Murre & Dros) found the same shape. The graduate's practical conclusion is the spacing effect: reviewing something just as it begins to fade slows the next fall. The toy below uses a simple exponential model with made-up constants, so it shows the shape of the idea, not your forgetting.

With no review, almost nothing is left by day 30 in this model (under 0.01%). With one review, about 2%. With four spaced reviews, about 87%. The numbers are invented; the reason spacing beats cramming is the finding.

Memory is rebuilt each time, not replayed

The folk picture is a video recording. The research picture, which goes back to Frederic Bartlett's 1932 Remembering, is a reconstruction: when you recall, you assemble the memory from fragments plus what you know about how things usually go. That makes memory efficient and error-prone, and the errors are not random. They move toward what makes sense.

A neat way to see it is the paradigm that James Deese (1959) and Henry Roediger and Kathleen McDermott (1995) turned into a standard tool. Study a list of words that all circle one unlisted word, for example bed, rest, tired, dream, nap, then later say which words you saw. People often "recognise" the unlisted word, here sleep, nearly as readily as words that were really on the list. Try it.

The same machinery explains Elizabeth Loftus's best-known results. In 1974 Loftus and John Palmer showed people films of car crashes and then asked how fast the cars were going when they "smashed into" or "contacted" each other. The "smashed" group guessed about 41 mph; the "contacted" group about 32. A week later, those asked with "smashed" were more likely to report broken glass that was never in the film. In 1995 Loftus and Jacqueline Pickrell planted a false childhood memory of being lost in a shopping mall, and about a quarter of the 24 participants reported remembering it. Critics have noted that "remembering" in these studies is partial, and that planting a whole memory is harder than planting a detail. The settled point is narrower and still important: suggestion after the event can change what people report, and they can't feel the change.

Eyewitnesses: sincere, confident and sometimes wrong

This is where memory science meets the law. In the United States, the Innocence Project reports eyewitness misidentification in about 69% of the first 367 DNA exonerations it counts (the share was about 75% in older snapshots, when there were fewer cases). Countries differ in how they run and weigh identifications, but human memory is the same everywhere. The American National Academy of Sciences' 2014 report Identifying the Culprit reviewed the evidence and recommended practices such as blind lineup administration and recording how confident a witness is at the first identification.

The fairest summary has two halves, and researchers disagree about their weights. Many studies show that confidence at trial, after months of repeated questioning, tells you little about accuracy. John Wixted and Gary Wells (2017) argued that the first confidence statement, from an uncontaminated lineup, is quite informative: high-confidence initial picks are usually right, and low-confidence picks often wrong. Both can be true, because "confident" at trial is not the same thing as "confident at the start".

To see the structure, borrow signal detection theory. A witness gets a familiarity signal for each face. The culprit's signal is stronger by an amount d′ (sensitivity). The witness also has a bar (the criterion): how familiar a face must feel before they pick it. A lineup puts the possible culprit among five look-alike fillers. When the culprit is absent, a witness who feels pressure to choose will still choose.

Notice what the sliders show. With d′ at 0, the witness knows nothing and the picks are noise. With a low bar, the witness picks someone nearly every time, and in culprit-absent lineups that someone is innocent. Raising the bar costs correct IDs but removes most of the false ones. That is why reformers focus on procedure: blind administration, fair fillers, and not telling the witness "good, you picked the suspect".

Heuristics: fast rules that mostly work, and sometimes don't

Amos Tversky and Daniel Kahneman's 1974 paper "Judgment under Uncertainty: Heuristics and Biases" showed that people judge probability by shortcuts. Availability: what comes easily to mind seems common, so after a news story people overestimate plane crashes. Representativeness: what resembles a type seems likely to belong to it, which is how the "Linda" problem (1983) leads many to rate "feminist bank teller" as more probable than "bank teller", though a subset can never be more probable than the whole. Anchoring: a number you've just heard drags your estimate toward it.

Gerd Gigerenzer and others argue that heuristics are not just errors: in the right environment, simple rules do well, and the "biases" are partly artefacts of how problems are posed. Kahneman's side says the errors are systematic and costly. Both agree that the way you present numbers matters. A classic case from Tversky and Kahneman involves a witness again.

With 15% Blue cabs and an 80% accurate witness, the chance the cab really was Blue is about 41%, not 80%. Out of 1,000 cabs, 150 are Blue and the witness calls 120 of them Blue; of 850 Green cabs, the witness wrongly calls 170 Blue. So 120 of 290 "Blue" reports are right. In Tversky and Kahneman's report, most people answered near the witness's 80%, ignoring the base rate. Counting people instead of multiplying probabilities (Gigerenzer's "natural frequencies") makes the right answer much easier to find.

Language and thought: does the word shape the idea?

Edward Sapir and Benjamin Lee Whorf are credited with the idea that language shapes thought. The strong version, that you cannot think what you cannot say, has been rejected: people think about things for which they have no word, and babies think before they speak. The weak version, that language nudges attention and memory, has support. Berlin and Kay (1969) showed that colour terms across languages follow a near-universal order, suggesting biology constrains the vocabulary. And Winawer and colleagues (2007) found that Russian speakers, whose language has separate basic words for light blue (goluboy) and dark blue (siniy), were quicker to tell apart two blues that straddle that line, an advantage that vanished when they had to repeat a string of digits at the same time. Other claims, such as grammatical gender making speakers describe objects differently, are more contested, and several famous language-and-thought results have been hard to replicate. Say "language nudges" and stop there.

The replication crisis touches this whole course. In 2015 the Open Science Collaboration tried to repeat 100 published psychology studies; about 97% of the originals had significant results and about 36% of the replications did. Cognitive-psychology effects, which are usually within-person and measured in milliseconds, fared better than social ones. Stroop, serial-position and false-memory effects replicate easily. Treat very large claims from small studies with caution, and see Research Methods in Psychology (35/19).

What a graduate just sees

• Attention is a narrow beam: what you did not attend to is not "seen but forgotten", it was never stored.
• Working memory holds about four chunks. Better chunking, not harder trying, is how people hold more.
• How deeply you process something when it goes in decides how easily it comes out.
• Review at the edge of forgetting beats re-reading in one sitting.
• Recall is reconstruction. A vivid, detailed, confident memory can still be wrong.
• Questions asked after an event can change the memory without any feeling of change.
• A witness's accuracy depends on procedure: how the lineup was run and how soon confidence was recorded.
• When someone gives you a probability, ask for the base rate; count cases instead of multiplying fractions.
• Language nudges thought; it does not cage it.

Where it shows up next

• Developmental Psychology (35/24): memory, attention and reasoning are not fixed. Children's working memory and theory of mind grow, and older adults' processing speed slows while vocabulary keeps growing.
• Behavioral Economics (34/46): the heuristics-and-biases programme is where much of the field began; its replication record is mixed, as the economics piece says.
• Machine Learning (34/32): human memory as a rebuild-from-cues system resembles models that fill gaps with likely content, which is both a strength and the source of confident errors.
• Law: Criminal Law & Procedure (35/07) and Civil Procedure & Evidence (35/08) meet eyewitness memory directly. Courts in different countries weigh identifications differently, and a number of jurisdictions have adopted lineup reforms.
• In the world: air traffic and cockpit design (attention and workload), interface design, teaching (spacing and retrieval practice), police interviewing, and the warnings on any "I remember it clearly" claim.

If you want to go deeper

• 📘 Textbook. Michael Eysenck and Mark Keane, Cognitive Psychology: A Student's Handbook (Routledge / Psychology Press). A common UK and international choice, thorough and research-heavy.
• 📘 Textbook. Daniel Reisberg, Cognition: Exploring the Science of the Mind (W. W. Norton). A common US choice, easier on the first read.
• 📘 Gentler. Daniel Schacter, The Seven Sins of Memory (Houghton Mifflin, 2001), and Daniel Kahneman, Thinking, Fast and Slow (2011), noting that some priming studies in the latter did not replicate. Elizabeth Loftus's TED talk "The fiction of memory" is short.
• 🌐 Free course. MIT OpenCourseWare 9.00SC Introduction to Psychology (Fall 2011), whose lectures include memory and cognition; the free Noba chapters cover each topic here.